Since a silicon carbide light-emitting diode was first developed, technological advances in its structure and fabrication method have progressed. Especially, a nitride light-emitting diode and various fluorescent materials have been developed in recent years, thereby implementing white light.
The nitride light-emitting diode has high chemical stability and a mechanism capable of generating blue light. Recently, the maximization of light efficiency and the improvement of heat dissipation during packaging have been discussed. Especially, various attempts to solve the above problems by modifying the shape of a chip and its fabrication process, besides the arrangement of a heat sink in a packaging process, have been made.
The light-emitting diodes are divided into a normal type light-emitting diode, a flip-chip type light-emitting diode, and a vertical type light-emitting diode.
The normal type light-emitting diode has a structure in which an electrode is formed on an n-type layer. However, it is necessary to partially etch a p-type layer and an active layer to expose the top of the n-type layer. Such a structure has the problem that current flows along the interface of the active layer in the vertical direction and flows in the lateral direction toward the n-type electrode. Moreover, the current density in the active layer adjacent to the n-type electrode is relatively higher than that of the other area. Especially, a sapphire substrate as a lower substrate remains in the chip process, and thus it causes problem that cannot effectively dissipate heat from a chip during the heat dissipation.
The flip-chip type light-emitting diode has a structure in which a ball-shaped bump is formed on the n-type electrode and the p-type electrode and is then bonded to a lower substrate. In general, the shape of the chip is the same as the normal type light-emitting diode. That is, the region where the n-type electrode is formed is produced by etching the p-type layer and the active layer. The flip-chip type light-emitting diode has a mechanism in which light is emitted toward the sapphire substrate and the heat dissipation is made through the metal ball-shaped bump formed at the bottom. Since the flip-chip type light-emitting diode includes the sapphire substrate having low thermal conductivity, it is difficult to obtain good heat dissipation properties.
The vertical type light-emitting diode is considered as a new alternative to solve the problems of the normal type and flip-chip type light emitting diodes.
According to the vertical type light-emitting diode, a nitride light-emitting diode is formed on a conventional non-conductive sapphire substrate, and an acceptor substrate is provided on the p-type layer or a reflective layer. Then, the sapphire substrate is separated, and an n-type electrode is formed on an n-type layer.
Especially, the problem occurred during the formation of the vertical type light-emitting diode is to separate the sapphire substrate, which is generally called a lift-off process in the art. The lift-off process employed at present uses a laser. The lift-off process using laser irradiates a laser beam to a gallium nitride layer formed on the sapphire layer. The irradiated laser beam is transmitted through the sapphire substrate and then absorbed in the gallium nitride layer. The absorbed energy separates the bonding of gallium nitride and generates N2 gas. The sapphire substrate and the gallium nitride layer are separated from each other by the generated N2 gas, and thus a predetermined region on the surface of gallium nitride is composed of metallic gallium, from which nitrogen is excluded.
The lift-off process using laser has the problem that the laser beam is completely absorbed in the gallium nitride layer. That is, the irradiated laser beam is transmitted through the gallium nitride layer to damage the crystalline structure of the active layer having a multi-quantum well (hereinafter referred to as MQW) structure. Moreover, a dislocation as a line defect may occur in the gallium nitride layer forming a buffer layer due to the energy of the laser beam. Especially, the dislocation is a progressive defect and, as the power is continuously supplied to the light-emitting diode, the dislocation becomes more severe.
To solve the problems occurring in the lift-off process, a method of controlling the thickness of the gallium nitride layer as the buffer layer or a method of controlling the energy of the laser beam has been proposed in the art. However, such methods cannot completely solve the problems occurring in the lift-off process using laser. Therefore, a method of fabricating a new vertical type light-emitting diode, which can prevent the damage of the gallium nitride layer and maintain the crystallinity of the active layer due to the irradiation of the laser beam, is required.